Method and apparatus for minimizing flue dust collections



Jan. 24, 1961 ARTER 2,969,047

METl-IOD AND APPARATUS FOR MINIMIZING FLUE DUST COLLECTIONS Filed July 19, 1954 3% /v VEN 701? 2 GL ENN A. ARTER.

BY a a/002040 #flaja/m ATTORNEYS United States atent O METHOD AND APPARATUS FOR MINIMIZING FLUE DUST COLLECTIONS Glenn A. Arter, 5125 N. Kenmore Ave., Chicago, Ill.

Filed July 19, 1954, Ser. No. 444,305

22 Claims. (CI. 122-379) This inventi'on relates to a method and to apparatus for substantially minimizing the collection upon normally magnetized members of magnetic particles entrained in a gaseous medium that fiows about the members. More especially, the invention is applicable to boilers for minimizing the collection upon the heat transfer surfaces thereof of ferrous particulate matter entrained in`the hot combustion gases that flow over the heat exchanger. While having particular utility in the environment of a boiler and especially in conjunction with the economizer or air heater thereof, it Will be apparent that the invention is useful in other settings where it is desirable to lessen the disposition from a gaseous medium of magnetic particles upon normally magnetized surfaces.

In the use of industrial boilers, the tendency is to make these structures more efficient and efiiciency is ordinarily increased by adding to the usual boiler apparatus an economizer that functions to extract heat from the hot combustion gases prior to their escape for the purposeof preheating the boiler water. Frequently air preheaters for heating combustion air are used in addition to economizers and in this event the combustion gases flow over heat exchangers which carry the air in its travel to the combustion chamber of the boiler. Economizers and air preheaters may be used individually or together and when used, serve the purpose of increasing boiler efficiency for they extract considerable heat from the hot combustion gases that otherwise would be lost in their escape to atmosphere.

Heretofore, there has been a practical minimum to which the temperature of the combustion gases could be lowered by heat extraction in economizers, air preheaters, etc. and that temperature has been above the dewpoint of at least some of the products of combustion in the gases and especially well above the dewpoint of the sulfur trioxide which forms a part of the combustion gases. If the temperature of the combustion gases is lowered to a point below the dewpoint of the sulfur trioxide, the sulfur trioxide condenses upon the surfaces of the heat exchanger tubes in the economizer or air preheater.

Once sulfur trioxide condenses upon metal surfaces, it combines with moisture from the combustion gases and sulfuric acid is formed. This acid, of course, reacts with the metal surfaces of the heat exchangers and causes corrosion. The sulfuric acid that forms on the metal surfiaces is in a liqnid state and it is found that particulate matter adheres to the wet surfaces and an ash collection commences. Eventually, the ash collection reaches such proportions that it plugs the flow of combustion gases and at such time the boiler must be shut down to clean the heat exchanger tubes.

The ash collection upon the heat exchange surfaces is a hard scale that is not easily removed and it collects quite rapidly and in many installations the shut down of the boiler is necessitated as frequently as each week'. Of course, heat exchange efiiciency is considerably hampered by the scale collections upon the heat ex- "ice changers and a collection of approximately one-eighth of an inch will result in a loss of etficiency of about 16%. Thus, even before the ash collections become so large that the flow of combustion gases is prevented, the efficiency of the heat exchangers has been sharply reduced.

In order to decrease the rate at which such scale collections occur, the combustion gases are permitted to escape at a temperature quite a bit higher than the dewpoint temperature of the sulfur trioxide. For example, the dewpoint of sulfur trioxide (which may vary somewhat from installation to installation) is about from 300 to' 350 F. and frequently the combustion gases are permitted to escape at 500 F. It has been determined that the efficiency of a boile-r may be increased by about 1% for every 35 to 40 decrease in the temperature of the combustion gases when they escape. Thus, a very substantial gain in efiiciency will be realzed if the temperature of the combustion gases could be lowered to the dewpoint of the sulfur trioxide or preferably lower.

The hot combustion gases contain small amounts of sulfur which appear as sulfur dioxide and sulfur trioxide, and entrained particles of iron and iron oxides. Also present in amounts dependent upon the chemical nature of the fuel are oxides of silicon, aluminum, calcium, magnesium, etc. The precise amount of iron oxide entrained in the combustion gases may vary considerably, but in general, constitutes a substantial amount, and in analyses that have been made of ash deposits, has ranged as high as 60%. Analysis of the scale that collects upon the heat exchangers indicates that sulfur trioxide and iron oxide form substantially the greatest part of the scale. In specific tests that have been made of scale from economizer tubes it has been found that from a practical standpoint the scale may be considered as being entirely magnetic and chemically composed of iron oxide and sulfur trioxide.

I have discovered that the collection of ash upon the heat exchanger tubes of economizers and air preheaters can be substantially minimized by demagnetizing or degaussing the heat exchangers. Typically, an economizer may have three banks of heat exchanger tubes, each bank spaced from the one next to it by a foot or so. The banks are interconnected and the coldest bank is the one adjacent the gaseous discharge end of the economizer. Usually, only that bank will have a temperature low enough to condense sulfur trioxide and that is the economizer bank that in its entirety I prefer to demagnetize.

The reason that demagnetizing or degaussing minimizes the collection of ash upon the heat exchangers s that each bank in an economizer is a large metal mass mem- 'ber and will have considerable residual or permanent magnetism and induced magnetism. The permanent magnetism may be taken on in numerous ways and, for example, may simply be the result of striking the heat exchanger tubes during Construction of the economizer. It is well known that one of the ways in which magnetism can be set up in a ferrous metal member is to strike it, for blows tend to orent the molecules under the influence of the earth`s magnetism so as to form the North and South Poles of a magnet. The induced magnetsm, on the other hand, is the result of the influence of the earth's magnetic field upon the metal members and is ever present unless the members are magnetically shielded. That induced magnetism is present in metal structures is well known and it has been proposed that the earth's magnetic field is distorted slightly by relatively large metal structures and this distortion nduces a magnetc field in a metal structure. In any event, by actual measurement it is found that a magnetic field does exist about each heat exchanger bank in economizers, etc. and I believe that it is this magnetic field which meam initially attracts the ferrous particulate matter entrained in the combustion gases to the wet surfaces of the economizer or air preheater, etc. heat exchanger tubes. If the heat exchanger surfaces have a wet film thereon, the magnetically attracted particles will clting thereto and eventually build up into a hard, thick scale that in time will block the flow of combustion gases.

As a result of my investigations, I propose to substantially minimize the collection of magnetically attracted particulate matter entrained in a fluid medium upon the metal surfaces of metal members exposed to the flow of the fluid and it is accordingly an object of this invention to achieve such results. Another object of the invention is in providing a method and apparatus for minimizing the collection upon normally magnetized metal mass members of magnetically attracted particulate matter entrained in a gaseous medium. Still another object is to provide apparatus and a method for substantially reducing the collection of magnetically attracted particles entrained in combustion gases upon the heat exchanger surfaces exposed to the combustion gases. Yet another object is that of demagnetizng heat exchanger members, such as those adapted for use in economizers and the like, to lessen the collection thereon of magnetically attracted particulate matter entrained in combustion gases. A further object is in providing a method for minimizing the collection of ferrous particulate' matter entrained in combustion gases upon the heat exchanger banks in an economizer or air preheater, etc. by degaussing at least one of the heat exchanger banks by developing in that bank a magnetic field that is substantially equal to and opposite to the permanent magnetic field thereof and the magnetic field thereof nduced by the earth's magnetism. Yet a further object 'is in quipping 'at least one bank of a heat exchanger, such as is present in'an economizer used 'with b'oiler's 'and 'the like, with induc'for's that are capable of p'roducing, when a direct current 'is passed therethrough, magnetic fields having horizontal and vertical Components that will cancel the induced and permanent magnetic fields in that structure. Additional objects and advantages will appear as the speciflcation proceeds.

Embodiments of the invention are illustrated in the accompanying drawing, in which- Figure l is a diagrammatic View of a typical boiler installation incorporatin'g rny invention; Fig. 2 isa broken vertical sectonal view of one bank of the heat exchanger in the economizer section of the apparatus shown in Fig. 1; Fig. 3 is a transverse sectional View taken on the line 3-3 of Fig. 2; and Fig. 4 is a perspe'ctive view of one of the tubes in the heat exchanger bank illustrated in Figs. 2 and 3 and which shows a modification of the invention.

The boiler apparatus illustrated in Fig. 1 is`for the most part diagrammatic and is intended to show a typical boiler equpped with an economizer section. It will be appreciated that the boiler in Fig, 1 is illustrative only of one type of apparatus with which the invention may be used.

The boiler is designated generally with the letter A and includes an outer shell that is generally fol-med of firebrick or some other material 'that will be substantially impervious to temperatures and gases within the boiler. The upper end of the shell 10, which is indicated generally at 11, may be consdered a gaseous flow column for it acts as a conductor or flow passage through which the combustion gases rise which are formed in the combustion of the fuel that takes place within the fire pot 12. The boiler may use oil or coal or co'ke as the fuel for the problems are substantially the same irrespective of which of these fuels is employed.

Mounted within the shell lt) and above 'the fire pot 12 is the evaporator or the boiler section 13. The section 13 will comprse a plurality of interconnected flow tubes through which water circulates and extracts heat from the combustion gases. Above the boiler 13 is a' superheater 14, which is operative to extract further heat from the rising combustion gases. Water is fed into the tank 15 and is pumped therefrom by a pump 16 through a distributing head 17 and into the boiler tubes 13. As shown in Fig. 1, the heated water is returned from the boiler 13 to the tank 15. The heated water, largc ly in the form of steam, is withdrawn from the upper portion of the tank 15 and circulates through the superheater 14 and to a steam outlet with which that unit is equpped.

To increase efliciency of the boiler by extracting more heat from the combustion gases, an economizer 18 is provided within the gaseous flow column 11 and above the super heater 14. ordinarly, the economizer 18 incorporates a heat exchanger that is broken down into a plurality of banks, such as are shown in Fig. 1, and which are designated with the numerals 19, 20 and 21. The heat exchanger banks each include a pluralty of `interconnected flow tubes and each bank is connected to the others so that a continuous flow path through the heat exchanger is provided. As shown, the water may enter the economizer through the valve 2.2 and may leave the economizer through a Valve 23 that is connected to the tank 15 along the lower portions thereof. Cold water may be fed into the economizer, but generally water that has been prevously heated into steam, expanded and thereafter con-densed, is fed back into the economizer for a further cycle of use.

Each of the heat exchanger banks 19, 20 and 21 will contain a plurality of flow conduits 24, as is shown in Fig. 3, and these conduits or tubes will be interconnected so' as to provide a continuous flow path. If desired, each of the tubes 24 may be equpped with "vertically extending fins 25 which will function to provide a larger contact area over which the combustion gases m'ust 'pass and, therefore, will extract more heat from these gases. The precise number of tubes mploy'ed will tle'p'end upon the size and capacity of the economizer and one or thote banks of heat exchangers may be employed in the economzer.

Shown in th`e drawing following the economizer is a preheater 24a having an inlet 24b, outlet 240 adapted to communicate with the combustion chamber, and heat exchange conduts 24d connecting the inlet and outlet, The air preheater ordinarly follows the economizer but physically may be positioned above or below it, etc. as is Well known in the art. The heat exch'anger tubes 24d are positioned within the gaseous flow column 11 and combustion air passes through these tubes in its travel to the combustion chamber 12. If an air preheater is employed it may be desirable to incorporate the invention theren, rather than in the economizer or possibly in both of these units. The problems encountered, however, and the method and apparatus for obviating them will be the same in either event.

The recycled water flowing into the economizer generally enters the upper heat exchanger bank 21, which is the coldest of the economizer banks, and flows progressively into the warmer banks 20 and 19. Thus, if only an economizer is employed, the bank 21 will have the lowest temperature of the heat exchanger banks in the column 11. However, if an air preheater is used also, since it follows the economizer, it will have the coldest heat exchanger in the column 11.

It should be brought out that boilers and the economizers used therewith are rather massive metal structures and in a single bank of the economizer there may be present as many as or more individual flow tubes. In a typical installation, each tube is 13 /2 feet long and each economizer bank is 4 feet high and 46 inches Wide. Three banks are provided and these' banks 'are spaced apart by 2 feet. The outer diameter of each of the conduits 24 is 2 inches and the tubes have 3 inch centers horizontally'artd 4 inch centers vertically. The tubes are thick walled tubes and it 'will -be apparent that a considei'able'anun't of metal is present in such'an economizer bank.

As the hot combustion gases rise through the flue or gaseous flow column 11, a certain amount of sulfur tn'oxide will condense first upon the coldest economizer bank 21. As explained hereinbefore, the sulfur trioxide unites with the moisture present and forms a wet film of sulfuric acid upon the tubes 24. The induced and permanent magnetic field of the economizer bank 21 draws the ferrous particulate matter to the tubes and these particles adhere to the wet surfaces. As the scaly collection increases, heat exchange within the bank 21 decreases and the water flowing into the bank 20 is at a lower temperature. Thus, eventually the temperature of the bank 20 is lowered to that of the sulfur trioxide dewpoint and the same process occurs within the bank 20. Eventually this cycle is repeated in the bank 19. Thus, after initial lowering of heat exchange efiiciencywithin the coldest heat exchanger bank 21, -all of the banks have an ash collection and scale built up thereon. Moreover, the ash also collects in the spaces 26 between each bank and complete blockage of the gaseous flow column occurs if the boiler is operated for a suficient length of time. Ordinarily, the boilers are shut down to clean the heat exchanger tubes before this condition occurs.

I propose to substantially minimize, if not completely avoid, the collection of magnetically attracted particulate matter entrained in the combustion gases upon the heat exchange surfaces in the banks 19, 20 and 21. This result is accomplished by degaussing or demagnetizing the banks so that they do not attract the particulate matter thereto. While each of the banks 19, 20 and 21 may be demagnetized, demagnetizing the bank 21 generally will be sufiicient. Demagnetization or degaussing is carried out by providing inductors about the bank 21 or about the individual conduits 24 thereof. For example, as is shown in Figs. 2 and 3, a conductor 27 may be spirally wound about the entire bank 21 and another conductor 28 may be wound longitudinally about the bank. I prefer to employ two separate conductors for in this way individual compensation may be made for the horizontal and vertical components of the induced and permanent magnetic field of the bank 21. The conductors 27 and 28 may be secured in position in any suitable manner. The conductors may be equipped with insulation which will wthstand the temperatures within the column 11 and certain plastcs have been found suitable for this purpose. In the specific illustration given, the conductor 27 is spirally wound about the entire bank 21, while the conductor 28 is wound longitudinally about the bank and is drawn against the support members 29 and 30 which serve as a mounting means for the conduits 24.

The precise number of turns in the conductor 27 and in the conductor 28 will depend upon the magnitude of the current fiowing through these conductors. In a permanent installation I employ a direct current and run the direct current individually through each of the conductors. If desired, `a variable resistor or rheostat 31 and a rheostat or variable resistor 32 may be provided in each of the separate circuits for controlling the flow of current therethrough. Meters 33 and 34 may also be employed and are of such nature that they are responsive to the magnetic field of the bank 21 and measure the strength thereof. Adjustment of the resistors can be made to nullify the magnetic field of the bank 21 and bring the resulting magnetic field of the bank, as indicated on the meters, as close to zero as is necessary. It is understood in the art that a magnetic field is developed by each of the conductors and these fields are substantially equal to and opposite to the magnetic field of the bank 21 and serve to cancel it out, whereby the bank is degaussed or demagnetized.

'A modified form of the invention is illustrated in Fig.

'4 and in this case each of the tubes 24 has coiled longi- 'tudinlly thereof a conductor 35 and also a conductor 36 which is wound spirally therealoug. This form of the invention may be employed where it is desired to demagnetize each of the tubes 24 individually. The conductors 35 and 36 may each be connected separately to a D.C. source and the variable resistor and meter arrangement, shown in Fig. l, may be employed.

Irrespective of the particular form of the invention utilized in any given application the result is to demagnetize or degauss the normally magnetized metal members which, because of their magnetism would ordinarily attract particulate iron and iron oxide particles, etc. 'entrained in the combustion gases. The demagnetization is accomplished by providing substantially equal and opposite magnetic fields which cancel out the permanent and induced magnetic fields that normally exist in the metal members. This demagnetization may be brought about in any suitable way, as for example, by applying a direct current to the coils illustrated and relating the magnitude of the current to the number of turns in the inductors. The metal members might be heated to the temperatures at which demagnetization takes place, although I prefer to use the coil and direct current arrangement shown and described.

Generally, it will be desirable to install conductors permanently about the heat exchanger bank or banks, although in certain installations wherein the heat exchangers are shielded from the earth's magnetism, a single demagnetization of the heat exchanger banks may be sufiicient to provide permanent demagnetization. In such event, portable demagnetization apparatus may be employed, and then, if desired, an alternating current of progressively decreasing magnitude could be employed to disorient the molecules in the metal and bring about the demagnetization of the metal. v

Generally where an economizer is employed and wherein the economizer has a plurality of heat exchanger banks, demagnetization of the coldest bank will prove sufiicient. However, in the event that it is desirable .to demagnetize each of the banks, this may be accomplshed in the manner heretofore described, and to indicate this condition in Fig. l, coils 40 are shown about the bank 20 and coils 41 about the bank 19. The coils 40 and 41 will be the same as the coils previously described with reference to bank 21 and may be provided about the bank in its entirety or about each of the tubes within the banks.

Occasionally a boiler structure will be used that does not employ an economizer but does have an air preheater. In that event, it may, of course, be desirable to demagnetize the air preheater. Even where an economizer is employed it may be desirable to demagnetize the air preheater for ordinarily the heat exchange members thereof will be at a lower temperature than the heat exchange members in the coldest bank of the economizer. Indicated in Fig. l are coils 42 about the heat exchanger tubes 24d of the air preheater. These coils may be either about the entire heat exchanger or about the individual tubes thereof, all as has been previously described.

Generally the heat exchange tubes in the superheater of a boiler will be at such a high temperature that the metal tubes cannot become magnetized. There are occasions, however, in certain classes of boilers in which the superheater has a temperature close to the demagnetization temperature of the metal (approximately 1400" F.). Where this condition is present, demagnetizaton of the superheater either by providing coils wound thereabout or about the tubes thereof may be a desirable feature, and in Fig. 1 the coils 43 indicate that the superheater may be demagnetized where required.

From what has been said, it will be apparent that temperature conditions within the apparatus may influence the character of the demagnetization that must be provided; that is stronger demagnetizng fields may be required in heat exchangers that are relatively cool, while fields of lesser strength may prove suflicint when 7 'the .heat exchangers are .at a higher temperature. Thus, Nertical differentiation of the applied field strength can be provided when required by simply relating the current strength and coil turns so as to give the desired field strength. The term "vertical difterentiation" is used to indicate change with respect to the flow path of the gases rather than to a purely physical arrangement.

Similarly, horizontal diflerentiation can be provided; .for example, the 'temperature of a heat exchanger bank at :the center portion of the gaseous flow column may be greater than the temperature at the ends of the heat ex- .changer bank. Thus, a 'field of greater strength may be required at the ends. It is contemplated that horizontal diflerentiation can be made (also vertical dflerentiation) and, for example, may include measuring the field strength at various sections in an economizer, both vertically and hor izont ally and along therewith a greater or lesser num ber of turns of wire may be provided in sections where respectively a greater or lesser field strength is needed. Another arrangement would be to provide a plurality of separate coils, each with an ndividually controlled current source and changes in magnitude of the current would then provide the desired field strength. The spacing of the 'coils in the superheater indicate field strength differentiation by coil spacing.

The fields of greatest concern -are the permanent fields and induced fields in the metal members caused largely bythe efiects of the earth's magneticfield thereon. How- ;ever, compensation may be made for whatever magnetic ..fields appear in the heat exchangers as a result of stray electric Currents that might be caused; for example, by the association of dis-similar metals and fields that might be induced from other external .sources.

For the sake of Simplicity and clarty, only a funda- :rnentel current control arrangement has been shown and described, but obviously refined Controls, automatically `.operable to alter current flow in response to changes in field strength, could be provided all as is .well known in the art. For example, electronic Controls, electric-me ,ch-anical control arrangements or mechanical or fluid pressure `control arrangements or combinations thereof may .be provided where automatic operation is a requirement.

While ,in the foregoirg specification an embodiment of the invention, both as to the method and structure have been set outin considerable detail for purposes of illustration, it will be apparent to those skilled in the art ;that numerous changes may be made in these details without departing from the spirit and principles of the invention.

I claim:

vl. In a method of reducing the deposition of magnetically attracted materials contained in a fluid flow stream upon the surface of a member normally having a magnetic field thereabout and being disposed in the path of such flow stream so'as to have such surface traversed thereby, the steps of developing a magnetic field about `said member, and controlling the orientation and magnitude of said developed magnetic field so as to substantially oppose and approximate in value such normal magnetic field for counteracting the same, whereby the attraction of such materials to said member is substantially reduced.

2. The method of claim l in which such fluid flow stream is essentially gaseous, and in which such materials are solid particles.

3. In a method of reducing the deposition of magnetically attracted paticles carried in a gaseous flow stream upon the surface of a member normally having both permanent and induced magnetic fields thereahout .and being disposed in the path of such flow stream so as to hav such surface traversed'thereby, the steps of devalue at least one of such normal magnetic fields for coun teracting the same, whereby the attraction of :such particles to said member is substantially reduced.

4. The method of claim 3 in which `such one field .com-

`prises the permanent field.

S. The method of .claim 3 in which such one fieldcomprises the induced field. r

6. The method of claim 3 in which the orientation and magnitude of said developed magnetic field is controlled so as to substanitally oppose and approximate in value the resultant of both such *permanent* and induced mag- 7. -In -a method of reducingwthe deposition .of magnetically attracted particles carried in a gaseous flow stream of combustion gas origin upon the surf-aces of a heat exchanger normally having a magnetc field thereabout and being disposed in the path of such flow stream so as to have its surfaces traversed thereby, thes-teps of developing a -magnetic field about said heat exchanger, and controlling .the orientation and magnitude of said .developed magnetic field so as to substantially oppose and approximate in value such normal magnetc field for counteracting the same, whereby the attraction of such particles to said heat exchangeris substantially reduced.

8. The method ofclaim 7 in which such gaseous flow stream has an elevated temperature relative to said heat exchanger, and in which such particles are ferrous.

9. The method of claim 7 in which such normal magnetc field comprises both :induced and .permanent fields,

;and ,in which said developed magnetic field substantially .heat exchanger tube, and controlling the orientaiton and magnitude of said developed magnetic field so as to substantially oppose and approximate `in value such normal magnetic field -for counteracting the same, whereby the attraction of such particles to said heat exchanger tube is substantially reduced.

1'1. The 'method of claim 10 'in which said combuston chamber comprises a component of a boiler structure, and in which said heat exchanger tube is in the economizer section of such boiler structure.

`12. The method of claim 10 in which said combustion chamber comprises a component of a boiler structure, and in which said heat exchanger tube is in the air preheater section of such boiler structure.

13. The method of claim 10 in which said combustion chamber comprises a component of a boiler structure, and in which said heat exchanger tube is in the superheater section of such boiler structure.

14. 'The method of claim 10 in which the step of developing a magnetic field about said tube comprses flowing an electric current about said tube, and in which the control and orientation of the developed magnetic field comprises controlling the flow path of the electric current and regulating the magnitude thereof.

15. The method of claim 14 in which such normal 'magnetic field has both horizontal and vertical com- .mate in value the horizontal and vertical components of such normal magnetic field,

16. In a boiler structure .having a casing providing a Vcombustion chamber thei-ein and definig a flow column communicating with said chamber for carrying combus` tion gases therefrom, heat transfer tubes mounted within said casing in the portion thereof defining said flow colunn, said heat transfer tubes having a magnetic field comprising residual and induced components, electric conductor means disposed about said tubes to provide a magnetic field in opposition to at least certain components of the residual and induced magnetic field of the tubes, and means for flowing an electric current through said conductors to create a magnetic field effective to substantially reduce the magnetic field of said tubes.

17. The structure of claim 16 in which electrc 0011- ductor means extend about all of said tubes.

18. The structure of claim 16 in which said electric conductor means compn'ses a plurality of conductors respecti'vely disposed about said tubes.

19. The structure of claim 16 in which the resultant of said residual and induced components has vertical and horizontal components, and in which said electric conductor means comprises a pair of conductors respectively coiled about the major and mnor dimensions of said tubes to substantially counteract such vertical and horizontal components.

20. In a boiler structure or the like having an economizer equipped with a plurality of heat transfer members oriented in banks and about which hot combustion gases flow to eflect a heat transfer therewith, means being provided to define a flow column for the gases, means for substantially cancelling the permanent and induced magnetic fields in the bank of heat exchangers adjacent the outlet of said economizer, said means comprising an electric conductor extending about that bank and disposed so as to produce a magnetic field having predominantly vertical components and an additional conductor disposed about that bank oriented to produce a magnetic field having predominantly horizontal components, and means for passing a controlled direct current through said conductors to produce magnetic fields of preselected strength.

21. In a boiler structure of the character described having an air pre-heater comprising a plurality of flow conduits about which hot combustion gases flow to effect heat transfer therewith, means being provided to define a flow column for the gases, means for substantially minimizing the efiects of the permanent and induced magnetic fields in the heat exchanger tubes, said means comprising an electric conductor extending about the tubes and being disposed so as to produce a magnetic field having vertical components and an additional conductor disposed about said tubes and being oriented to produce a mag netic field having horizontal components, and means for passing a direct current of preselected magnitude through said conductors to produce magnetic fields of preselected strength and phase to cancel the magnetic field of said tubes.

22. In a boiler structure equipped with a super-heater having heat exchange members disposed in the flow path of the hot combustion gases of said boiler, means being provided to define a flow column for the gases, means for substantially cancelling the effects of the permanent and induced magnetic fields of said heat exchanger comprising a first conductor oriented to produce a magnetic field having vertical components, an additional conductor oriented with respect to said heat exchanger to produce a field having horizontal components, and means for flowing a current of preselected strength through said conductors to produce fields substantially equal to and opposite in phase to the permanent and induced magnetic fields of said heat exchanger.

References Cited in the file of this patent UNITED STATES PATENTS 59,910 Hay Nov. 20, 1866 2,421,583 Stuart June 3, 1947 2,449,946 Martin et al Sept. 21, 1948 2,460,684 Farrow Feb. 1, 1949 2,469,635 Dalin et al May 10, 1949 

